1. Field of the Invention
The present invention relates to a multicolor image forming apparatus, and more particularly to a multicolor image forming apparatus suitable for preventing color slippage when a multicolor image is obtained by superimposing a plurality of monochromatic toner images.
2. Description of the Related Art
In image forming apparatus such as copying machines, facsimiles, printers and the like utilizing electrophotography, the surface of an image carrier uniformly charged with electricity by a charger is irradiated with laser beam so as to form an electrostatic latent image and a development device is then used to convert this latent image into a toner image. Then, this toner image is transferred directly or indirectly onto a printing medium (hereinafter referred to as "recording paper") such as paper and plastic film sheets before being outputted as a printed image.
A typical multicolor image forming apparatus is provided with two image carriers and a plurality of monochromatic toner images formed on the first image carrier are superimposed on the second image carrier (hereinafter referred to as "intermediate transfer body") so as to form a multicolor toner image, which is then transferred onto recording paper.
In order to transfer the multicolor toner image from the intermediate transfer body onto the recording paper as the secondary transfer in a multicolor image forming apparatus of the sort described above, a transfer roll is provided opposite to the intermediate transfer body. This transfer roll is pressed against the intermediate transfer body only at the time of secondary transfer and separated therefrom at the time of other than the secondary transfer. Consequently, the intermediate transfer body is subjected to shock when the transfer roll is pressed against the intermediate transfer body and this may cause color slippage of the image.
In the case of an electrophotographic apparatus as disclosed in Japanese Patent Unexamined Publication No. Hei. 5-72912, torque is given to the transfer roll from a driving source through a so-called one-way clutch capable of transmitting the torque only in one direction and the peripheral speed of the transfer roll is set to be lower than the speed of the intermediate transfer body. The clutch is released and the transfer roll is urged by the intermediate transfer body to undergo so-called free-running because the speed of the intermediate transfer body is greater than the peripheral speed of the transfer roll when the transfer roll is brought into contact with the intermediate transfer body. Consequently, the peripheral speed of the transfer roll becomes equal to the speed of the intermediate transfer body, whereupon it is anticipated that the shock received by the intermediate transfer body is eased when both are brought into contact with each other.
The following problem is posed in the aforesaid conventional apparatus. The free-running of the transfer roll caused by the intermediate transfer body means that the self-inertia acceleration torque and the frictional torque of the transfer roll act on the intermediate transfer body. Therefore, the load of the intermediate transfer body will differ between the time when the transfer roll is kept in contact with the intermediate transfer body and the time other than the former. In other words, the load fluctuates between the secondary transfer to the recording paper and the time other than the secondary transfer. On the other hand, the transfer of the monochromatic toner image from the first image carrier to the intermediate transfer body, that is, the primary transfer, is carried out irrespective of whether or not the secondary transfer is being carried out. Consequently, the problem is that the monochromatic toner image subjected to the primary transfer during the secondary transfer may shift in position from the monochromatic toner image subjected to the primary transfer not during the secondary transfer, resulting in color slippage of the multicolor image. Since the cleaning blade is pressed against the transfer roll in order to clean the surface of the transfer roll, the load derived from the frictional resistance of the cleaning blade also constitutes the load applied to the intermediate transfer body indirectly and this also causes color slippage to the multicolor image like the above-described problem.
A cleaning means, that is, a cleaner for removing residual toner (waste toner) on the intermediate transfer body is provided and when the cleaner is pressed against the intermediate transfer body, the load fluctuation of the intermediate transfer body is produced. A brush-like cleaner may be considered in order to lower the frictional resistance between the cleaner and the intermediate transfer body but this also results in the problem of not only complicating the mechanism but also increasing the cost.